Gasoline engines produce combustion exhaust streams containing hydrocarbons, carbon monoxide, and oxides of nitrogen in conjunction with particulates. It is known to treat the gases with a three-way catalyst composition, and it is known to recover the particulates in particulate traps such as soot filters.
Historically, gasoline engines which are operated predominantly stoichiometrically have been designed such that low levels of particulates were formed. However, gasoline direct injection (GDI) engines, which are finding increasing application due to their fuel efficiency, can have lean burn conditions and stratified combustion resulting in the generation of particulates. Particulate emissions for engines fuelled by gasoline fuel, such as gasoline direct injection engines, are being subject to regulations and existing after-treatment systems for gasoline engines are not suitable for achieving the proposed particulate matter standard.
In contrast to particulates generated by diesel lean burning engines, the particulates generated by gasoline engines tend to be finer and at lower levels. This is due to the different combustion conditions of a diesel engine as compared to a gasoline engine. For example, gasoline engines run at a higher temperature than diesel engines. Also, the resultant hydrocarbon components are different in the emissions of gasoline engines as compared to diesel engines.
Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide pollutants continue to become more stringent. In order to meet such standards, catalytic converters containing a three-way catalyst (TWC) are located in the exhaust gas line of gasoline-fuelled internal combustion engines. Such catalysts promote the oxidation by oxygen and oxides of nitrogen in the exhaust gas stream of unburned hydrocarbons and carbon monoxide, as well as the concomitant reduction of nitrogen oxides to nitrogen.
Emission legislation in Europe from 1 Sep. 2014 (Euro 6) requires control of the number of particles emitted from both diesel and gasoline (positive ignition) passenger cars. For gasoline EU light duty vehicles the allowable limits are: 1000 mg/km carbon monoxide; 60 mg/km nitrogen oxides (NOx); 100 mg/km total hydrocarbons (of which ≤68 mg/km are non-methane hydrocarbons); and 4.5 mg/km particulate matter ((PM) for direct injection engines only). The Euro 6 PM standard will be phased in over a number of years with the standard from the beginning of 2014 being set at 6.0×1012 per km (Euro 6) and the standard set from the beginning of 2017 being 6.0×1011 per km (Euro 6c). In a practical sense, the range of particulates that are legislated for are between 23 nm and 3 μm.
In the United States, on 22 Mar. 2012, the State of California Air Resources Board (CARB) adopted new Exhaust Standards from 2017 and subsequent model year “LEV III” passenger cars, light-duty trucks and medium-duty vehicles which include a 3 mg/mile emission limit, with a later introduction of 1 mg/mi possible, as long as various interim reviews deem it feasible.
The new Euro 6 (Euro 6 and Euro 6c) emission standard presents a number of challenging design problems for meeting gasoline emission standards. In particular, how to design a filter, or an exhaust system including a filter, for reducing the number of PM gasoline (positive ignition) emissions, yet at the same time meeting the emission standards for non-PM pollutants such as one or more of oxides of nitrogen (NOx), carbon monoxide (CO) and unburned hydrocarbons (HC), all at an acceptable back pressure, e.g. as measured by maximum on-cycle backpressure on the EU drive cycle.
It is known in gasoline systems to provide a three-way catalyst (TWC) located on a substrate carrier, such as a flow-through monolith. It is also known to combine the TWC and particulate removal functions in a single device by coating a TWC onto a wall-flow monolith (particulate filter). An example is described in US 2009/0193796.
Accordingly, it is desirable to provide an improved particulate filter and/or tackle at least some of the problems associated with the prior art or, at least, to provide a commercially useful alternative thereto.
US 2010/275579A1 discloses a catalytically active particulate filter, an exhaust gas cleaning system and a process for cleaning the exhaust gases of predominantly stoichiometrically operated internal combustion engines, which are said to be suitable for removing particulates from the exhaust gas, as well as the gaseous CO, HC and NOx pollutants also. The particulate filter comprises a filter body and a catalytically active coating consisting of two layers. The first layer is in contact with the incoming exhaust gas, the second layer with the outgoing exhaust gas. Both layers contain alumina. The first layer contains palladium. The second layer contains, in addition to rhodium, an oxygen-storing cerium/zirconium mixed oxide.
US 2009/087365A1 discloses a catalytically active particulate filter, an exhaust gas cleaning system and a process for cleaning the exhaust gases of predominantly stoichiometrically operated internal combustion engines, which are said to be suitable for removing particulates from the exhaust gas, as well as the gaseous CO, HC and NOx pollutants also. The particulate filter comprises a filter body and a catalytically active coating consisting of two layers. Both layers contain alumina. The first layer contains palladium. The second layer contains rhodium. The second layer is disposed above the first layer.
WO 2011/133503 discloses exhaust systems and components suitable for use in conjunction with gasoline engines to capture particulates in addition to reducing gaseous emission such as hydrocarbons, nitrogen oxides, and carbon monoxides. Exhaust treatment systems comprising a three-way conversion (TWC) catalyst located on a particulate filters are provided. Coated particle filters having washcoat loadings in the range of 1 to 4 g/ft3 are said to result in minimal impact on back pressure while simultaneously providing TWC catalytic activity and particle trapping functionality sufficient to meet Euro 6 emission standards. Relatively high levels of oxygen storage components (OSC) are said to be delivered on and/or within the filter. However, it is not possible from the information provided to determine a weight ratio of OSC:alumina, except in one embodiment, wherein the TWC catalytic material is substantially free of alumina, i.e. the ratio of OSC:alumina is ∞. The filters can have a coated porosity that is substantially the same as its uncoated porosity. The TWC catalytic material can comprise a particle size distribution such that a first set of particles has a first d90 particle size of 7.5 μm or less and a second set of particles has a second d90 particle size of more than 7.5 μm.
WO 2014/125296 discloses a positive ignition engine comprising an exhaust system for a vehicular positive ignition internal combustion engine, which exhaust system comprising a filter for filtering particulate matter from exhaust gas emitted from the vehicular positive ignition internal combustion engine, which filter comprising a porous substrate having inlet surfaces and outlet surfaces, wherein the porous substrate is coated at least in part with a three-way catalyst washcoat comprising a platinum group metal and a plurality of solid particles, wherein the plurality of solid particles comprises at least one base metal oxide and at least one oxygen storage component which is a mixed oxide or composite oxide comprising cerium, wherein the mixed oxide or composite oxide comprising cerium and/or the at least one base metal oxide has a median particle size (D50); less than 1 μm and wherein the platinum group metal is selected from the group consisting of: (a) platinum and rhodium; (b) palladium and rhodium; (c) platinum, palladium and rhodium; (d) palladium only; or (e) rhodium only.